The present disclosure relates to a novel method to adjust the melt flow index (MFI) of a toner by pH adjustment.
The melt flow index (MFI) of a toner is a measurement of the flow of the toner when it is in the fluid state. It is also a surrogate measurement of viscosity and elasticity used in the manufacturing environment. The flow of a toner is an important property because it drives the interaction between the toner and the fusing subsystem components of printers and xerographic devices. If the MFI of the toner is not within specification, the image quality can suffer due to defects such as spots, streaks and smudges. These defects can be the result of the toner not adhering enough to the substrate, toner not melting completely, toner sticking to the fuser roll, and also excessive contamination of the cleaning web.
It has been believed that the MFI of an emulsion aggregation (EA) parent particle is dictated by the formulation of that particle and that the variability of that MFI is due largely to the variation of the raw materials. This variability leads to either very wide specifications that test or exceed the boundaries of machine performance (fusing, Additive Adhesion Force Disruption, which measures how well external additives are attached to the particle, aging) and blocking. If the specification limits are not wide enough, this leads to increased testing, increased inventory during testing, and increased product loss. So far, the control of the MFI of a toner is done by setting specifications for raw materials properties, in particular the latex. Some of the key latex properties that drive the MFI include molecular weight, glass transition temperature, and also the crosslinking density. In addition, the MFI of a toner may be adjusted by altering the amount of flocculent in the particle formation process. However, these strategies can lead to raw material specifications that may be difficult to meet, and can also lead to changes in process outputs such as coarse generation and aggregation rate.
There remains a need to be able to control and adjust the MFI of a toner that reduces the variability while limiting the amount of testing and product loss.
Numerous processes are within the purview of those skilled in the art for the preparation of toners. Emulsion aggregation is one such method. These toners are within the purview of those skilled in the art and toners may be formed by aggregating a colorant with a latex polymer formed by emulsion polymerization. For example, U.S. Pat. No. 5,853,943, the disclosure of which is hereby incorporated by reference in its entirety, is directed to a semi-continuous emulsion polymerization process for preparing a latex by first forming a seed polymer. Other examples of emulsion/aggregation/coalescing processes for the preparation of toners are illustrated in U.S. Pat. Nos. 5,403,693, 5,418,108, 5,364,729 and 5,346,797, the disclosures of each of which are hereby incorporated by reference in their entirety. Other processes are disclosed in U.S. Pat. Nos. 5,527,658, 5,585,215, 5,650,255, 5,650,256 and 5,501,935, the disclosures of each of which are hereby incorporated by reference in their entirety.
Improved methods for producing toner, which decrease the production time and permit excellent control of the charging of toner particles, remain desirable.